Photochromic glass with sharp  cutoff

ABSTRACT

A photochromic glass that includes a base glass and a photochromic agent is described. The base glass is a modified boroaluminosilicate glass and the photochromic agent is a nanocrystalline cuprous halide phase. The photochromic glass exhibits a sharp cutoff in the UV or short wavelength visible portion of the spectrum along with an absorption band at longer wavelengths in the visible. The nanocrystalline cuprous halide phase includes Cu 2+ , which provides states within the bandgap of the cuprous halide that permit the glass to absorb visible light. Absorption of visible light drives a photochromic transition without compromising the sharp cutoff. The nanocrystalline cuprous halide phase may optionally include Ag.

RELATED APPLICATION INFORMATION

This application is a divisional of U.S. application Ser. No.15/354,470, filed Nov. 17, 2016, which claims priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 62/269,285 filed on Dec.18, 2015, the contents of which are relied upon and incorporated hereinby reference in their entirety.

FIELD

This description pertains to photochromic materials. More particularly,this description pertains to photochromic glass with strong absorptionof light in the UV or short wavelength visible portions of the spectrum.Most particularly, this description pertains to photochromic glasshaving a sharp absorption feature near 400 nm and a photochromicresponse capable of being initiated by visible light.

BACKGROUND

Photochromic glass has been widely used in the field of ophthalmiclenses with a particular emphasis on applications to sunglasses. Uponexposure to actinic radiation, a photochromic glass undergoes aphotochemical or photostructural transformation that leads to darkeningand a reduction in the transmission of light in a particular spectralrange. In the case of sunglasses, the actinic radiation may be sunlightand the photochromic response may lead to a reduction in transmission ofvisible light to reduce the intensity of light that reaches the eye. Thephotochromic response protects the eye from unsafe intensity levels andprovides comfort to the wearer.

The earliest commercially successful photochromic glasses utilizedsilver halide crystals as the photochromic agent. The silver halides canbe incorporated into a variety of base glasses to provide a photochromicglass suitable for ophthalmic uses. Typical base glasses arealkali-doped silica or modified silica glasses. U.S. Pat. No. 3,208,860(Armistead and Stookey) and U.S. Pat. No. 3,197,296 (Eppler andStookey), for example, describe photochromic glasses that include silverhalides in an alkali-doped boroaluminosilicate glass. Silver halidephotochromic glasses remain a viable commercial product and are sold byCorning, Inc. under the PHOTOGRAY® and PHOTOBROWN® product lines.

Silver halides have certain drawback as photochromic agents. First,silver halides have high sensitivity to actinic radiation and impart astrong photodarkening response when exposed to low levels of actinicradiation. As a result, a viewer wearing sunglasses made from silverhalide photochromic glass perceives little difference in the level ofdarkening observed at low (e.g. dawn) and high (e.g. mid-day) levels ofillumination. It would be desirable to develop a photochromic glass thatexhibits a more uniform photodarkening response, as perceived by theviewer, over the range of illumination intensities normally encounteredin daily activity. Second, the photodarkening response of silver halideglasses is sensitive to ambient temperature. For a given level ofillumination, photodarkening is more pronounced at low temperatures thanat high temperatures. This leads to seasonal variations in thephotochromic response that may be undesirable for many consumers. Third,silver is a relative expensive additive for glass and the cost of silverhalide photochromic glass limits the range of commercial applications.

Deficiencies in the performance and cost of silver halide glasses havemotivated interest in developing silver-free photochromic glasses forcommercial applications. U.S. Pat. No. 3,325,299 (Araujo); U.S. Pat. No.3,954,485 (Seward and Tick); and U.S. Pat. No. 4,166,745 (Araujo andTick) describe a series of photochromic glasses that use copper-cadmiumhalides as the photochromic agent. The copper-cadmium halides exhibitgood photochromic response and overcome many of the deficienciesassociated with silver halides: (1) batch cost is reduced by eliminatingsilver, (2) the photochromic response is less sensitive to ambienttemperature, and (3) a photodarkening response that varies moregradually with illumination intensity. Despite the advantages of thecopper-cadmium photochromic glasses, commercial prospects are limiteddue to concerns over toxicity and disposal of cadmium.

U.S. Pat. No. 3,325,299 (Araujo) also discloses glasses with copperhalides as the photochromic agent. The glasses are free of cadmium. Thecompositions, however, proved difficult to process and exhibited a hazyappearance that was unsuitable for most commercial applications.Haze-free photochromic glass compositions using copper halides as thephotochromic agent were presented in U.S. Pat. No. 4,222,781 (Morse andSeward). The base glass composition was a B₂O₃—Al₂O₃—SiO₂ glass withalkali dopants in addition to copper and halides. Preferred compositionsincluded low concentrations (˜1 wt %) of WO₃ or MoO₃ to improve thephotochromic response of the copper halide. Good photochromic response(darkening and fading) was observed for the glasses.

As the market for ophthalmic glasses has expanded, greater demands havebeen placed on the performance of photochromic glass. One concern is therecognition that overexposure of the eye to UV light is harmful. Thisconcern has motivated interest in developing photochromic glasses thateffectively filter UV light without compromising transmission in thevisible. The silver halide, copper-cadmium halide, and copper halidephotochromic glasses discussed above have absorption bands in the UV,but the absorption bands lack a sharp UV cutoff and the glasses transmita significant amount of UV light. U.S. Pat. No. 5,281,562 (Araujo andMorgan) discloses copper halide glasses having sharp UV cutoff near 400nm. The patent demonstrated that the shape and intensity of UVabsorption bands depends on the concentration of copper in the glass. Aseries of compositions with sharp UV cutoffs were disclosed. The glassesdescribed in U.S. Pat. No. 5,281,562 (Araujo), however, are notphotochromic and lack the reversible photodarkening response desired forophthalmic and other consumer applications.

There is a need for glasses having a sharp UV cutoff and a photochromicresponse.

SUMMARY

A photochromic material that includes a glass with a nanocrystallinephase is described. The glass is a modified boroaluminosilicate glassthat includes a nanocrystalline cuprous halide phase. The photochromicmaterial exhibits a sharp cutoff in the UV or short wavelength visibleportion of the spectrum along with an absorption band at longerwavelengths in the visible. The nanocrystalline cuprous halide phaseincludes Cu²⁺, which provides states within the bandgap of the cuproushalide that permit the material to absorb visible light. Absorption ofvisible light drives a photochromic transition without compromising thesharp cutoff. The nanocrystalline cuprous halide phase may optionallyinclude Ag.

The present disclosure extends to:

A photochromic glass comprising:

a base glass, said base glass including 19 wt %-39 wt % B₂O₃, 0.5 wt%-15 wt % Al₂O₃, 46 wt %-65 wt % SiO₂, 3.5 wt %-12.5 wt % alkali metaloxide, and 0.005 wt %-0.40 wt % SnO₂; and

0.56 wt %-1.28 wt % copper; and

0.1 wt %-0.7 wt %. halide; and

a photochromic agent, said photochromic agent including ananocrystalline cuprous halide phase within said base glass, saidnanocrystalline cuprous halide phase including a copper component and ahalide component, said copper component including at least a portion ofsaid copper, said at least portion of said copper including Cu⁺ andCu²⁺, said halide component including at least a portion of said halide;

wherein said photochromic glass has a cutoff wavelength less than 410nm.

The present disclosure extends to:

A method of making a photochromic glass comprising:

melting a batch composition, said batch composition including 19 wt %-39wt % B₂O₃, 0.5 wt %-15 wt % Al₂O₃, 46 wt %-65 wt % SiO₂, 3.5 wt %-12.5wt % alkali metal oxide, 0.40 wt %-2.5 wt % CuO, and 0.01 wt %-1.3 wt %halide.

The present disclosure extends to:

A photochromic glass comprising:

a base glass, said base glass including 19 wt %-39 wt % B₂O₃, 0.5 wt%-15 wt % Al₂O₃, 46 wt %-65 wt % SiO₂, 3.5 wt %-12.5 wt % alkali metaloxide, and 0.005 wt %-0.40 wt % SnO₂; and

0.64 wt %-1.12 wt % copper; and

0.1 wt %-0.7 wt %. halide; and

a photochromic agent, said photochromic agent including ananocrystalline cuprous halide phase within said base glass, saidnanocrystalline cuprous halide phase including a copper component and ahalide component, said copper component including at least a portion ofsaid copper, said at least portion of said copper including Cu⁺ andCu²⁺, said halide component including at least a portion of said halide;and

0.005 wt %-0.05 wt % Ag; and

0.05 wt %-0.50 wt % WO₃.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated in and constitute a part of thisspecification. The drawings are illustrative of selected aspects of thepresent description, and together with the specification serve toexplain principles and operation of methods, products, and compositionsembraced by the present description. Features shown in the drawing areillustrative of selected embodiments of the present description and arenot necessarily depicted in proper scale.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the written description,it is believed that the specification will be better understood from thefollowing written description when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts an absorption spectrum of a non-photochromic glassexhibiting excitonic absorption related to a cuprous halidenanocrystalline phase, an absorption spectrum of a non-photochromiccopper-containing glass that lacks the excitonic absorption feature andan absorption spectrum of a non-photochromic glass without copper thatlacks the excitonic absorption feature.

FIG. 2 depicts an absorbance spectrum of two glasses that exhibitexcitonic absorption and absorption due to the presence of Cu²⁺ in thecuprous halide nanocrystalline phase, where one of the glasses (dashedline) includes WO₃ in the composition and the other glass (solid line)does not.

FIGS. 3A, 3B, and 3C, respectively, show the transmittance of light froma broadband source, with and without a 400 nm longpass filter, through aphotochromic glass containing copper halide nanocrystals that has beensubjected to heat treatment at 500° C. for 2 hr, 12 hr, and 24 hr.

FIG. 4 depicts the absorption spectra of two glasses containing copperand halides, only one of which has a base glass composition suitable forstabilizing a copper halide nanocrystalline phase.

FIG. 5 shows absorption spectra of Cu-halide photochromic glasses havinga thickness of ˜200 μm in the vicinity of the excitonic absorptionfeature.

FIG. 6A shows absorption spectra of a series of photochromic glasseshaving cuprous halide nanocrystalline phases with various relativeproportions of Cl⁻ and Br⁻.

FIG. 6B shows an enlargement of the spectra shown in FIG. 6A in thevicinity of the cutoff wavelength.

FIG. 7 shows the variation in UV cutoff wavelength as a function of therelative amounts of Br⁻ and Cl⁻ in the cuprous halide nanocrystallinephase of photochromic glasses.

FIGS. 8A and 8B, respectively, compare the photodarkening response ofphotochromic glasses having a cuprous halide nanocrystalline phase withAg and without Ag.

FIGS. 9A-9C, respectively, illustrate the photodarkening response of aphotochromic glass having a cuprous halide nanocrystalline phase at 72°F., 122° F., and 160° F. and FIG. 9D shows an Arrhenius plot of theabsorption associated with the photodarkening response.

FIGS. 10A and 10B, respectively, depict the fading response ofphotodarkened glasses with Ag and without Ag in the cuprous halidenanocrystalline phase.

The embodiments set forth in the drawings are illustrative in nature andnot intended to be limiting of the scope of the detailed description orclaims. Whenever possible, the same reference numeral will be usedthroughout the drawings to refer to the same or like feature.

DETAILED DESCRIPTION

Reference will now be made in detail to illustrative embodiments of thepresent description.

The present specification describes photochromic glasses having variouscompositions. The glass compositions include various components presentin various concentrations. The glasses are formed by melting and coolinga batch composition that includes the components of the glass. As usedherein, concentration refers to the concentration of a component in thebatch composition. Due to differences in volatility of differentcomponents, the concentration of a component in the glass formedfollowing melting and cooling of the batch composition may differslightly from the concentration of the component in the batchcomposition.

The present specification describes photochromic glasses that exhibitgood photochromic response and a sharp cutoff at a UV wavelength or at ashort wavelength visible wavelength. The photochromic response can beinitiated by a wide range of wavelengths and exhibits favorabledarkening and fading characteristics. When the photochromic response hasbeen initiated and is manifest, the glass may be referred to as being inits photochromic or darkened state. When the photochromic response hasfaded or is not manifest, the glass may be referred to as being in itsnon-photochromic or faded state. The photochromic response transformsthe glass from its non-photochromic state to its photochromic state. Thephotochromic response may also be referred to herein as a photodarkeningresponse or a darkening response.

The photochromic glasses include a base glass and a photochromic agent.The base glass is an alkali-modified boroaluminosilicate glass. The baseglass optionally includes a redox agent. The photochromic agent includescopper and a halide. The photochromic agent optionally includes silver,tungsten, and/or molybdenum.

The base glass is a B₂O₃—Al₂O₃—SiO₂ glass with alkali metal oxidemodifiers. The B₂O₃ concentration is in the range from 18 wt %-41 wt %,or in the range from 19 wt %-39 wt %, or in the range from 20 wt %-37 wt%, or in the range from 22 wt %-35 wt %, or in the range from 24 wt %-33wt %, or in the range from 26 wt %-31 wt %. The Al₂O₃ concentration isin the range from 0.7 wt %-15 wt %, or in the range from 1.0 wt %-13 wt%, or in the range from 2.0 wt %-11 wt %, or in the range from 3.5 wt%-10 wt %, or in the range from 5.0 wt %-9.0 wt %, or in the range from6.0 wt %-8.0 wt %. The SiO₂ concentration is in the range from 46 wt%-65 wt %, or in the range from 48 wt %-63 wt %, or in the range from 50wt %-61 wt %, or in the range from 52 wt %-59 wt %. The alkali metaloxide content is in the range from 3.5 wt %-12.5 wt %, or in the rangefrom 4.0 wt %-11.5 wt %, or in the range from 4.5 wt %-10.5 wt %, or inthe range from 5.0 wt %-9.5 wt %, or in the range from 6.0 wt %-8.5 wt%. The alkali metal oxide content may include one or more of Li₂O, Na₂O,or K₂O. In one embodiment, Na₂O is the sole alkali metal oxide in theglass.

The base glass may optionally include a redox agent to influence theoxidation state of elements in the glass. The redox agent is an oxide ofan element capable of transforming between two or more oxidation states.Representative redox agents include As₂O₃, As₂O₅, Sb₂O₃, SnO, and SnO₂.In one embodiment, the redox agent is SnO, SnO₂, or a combinationthereof and the glass composition is free of As and Sb. Theconcentration of redox agent may be in the range from 0 wt %-1 wt %, orin the range from 0 wt %-0.80 wt %, or in the range from 0 wt % to 0.60wt %, or in the range from 0 wt % to 0.40 wt %, or in the range from0.005 wt %-0.60 wt %, or in the range from 0.005 wt %-0.40 wt %, or inthe range from 0.005 wt %-0.25 wt %, or in the range from 0.01 wt %-0.50wt %, or in the range from 0.01 wt %-0.40 wt %, or in the range from0.01 wt %-0.25 wt %, or in the range from 0.05 wt %-0.30 wt %.

The photochromic agent includes a nanocrystalline cuprous halide phasethat includes a copper component and a halide component. The coppercomponent includes a combination of Cu⁺ and Cu²⁺. The halide componentincludes one or more of the halides F⁻, Cl⁻, Br⁻, and I⁻. Thenanocrystalline cuprous halide phase may include two or more halides.The two or more halides may include Cl⁻ and Br⁻. The copper componentcan be incorporated by including CuO in the glass and heat treating theglass to form a nanocrystalline cuprous halide phase. The concentrationof CuO in the glass may be in the range from 0.40 wt %-2.5 wt %, or inthe range from 0.50 wt %-2.0 wt %, or in the range from 0.60 wt %-1.8 wt%, or in the range from 0.70 wt %-1.6 wt %, or in the range from 0.80 wt%-1.4 wt %, or in the range from 0.90 wt %-1.2 wt %.

The concentration of copper (Cu (in all oxidation states)) in the glassmay be computed from the concentration of CuO in the glass bymultiplying by the ratio of the molecular weight of Cu (63.55 g/mol) tothe molecular weight of CuO (79.55 g/mol). The concentration of coppermay be in the range from 0.32 wt %-2.00 wt %, or in the range from 0.40wt %-1.60 wt %, or in the range from 0.48 wt %-1.44 wt %, or in therange from 0.56 wt %-1.28 wt %, or in the range from 0.64 wt %-1.12 wt%, or in the range from 0.72 wt %-0.96 wt %. The relative proportions ofCu⁺ and Cu²⁺ in the glass depend on the melting conditions and ambientas well as the presence and concentration of redox agent.

At least a portion of the total copper (Cu) content of the photochromicmaterial is present in the nanocrystalline cuprous halide phase. Somecopper (Cu) is also present in the base glass portion of thephotochromic material.

Halide can be introduced to the photochromic glass by including alkalihalides in the batch composition used to form the glass. Representativealkali halides include NaCl and NaBr. The total halide concentration inthe glass may be in the range from 0.01 wt %-1.3 wt %, or in the rangefrom 0.01 wt %-1.2 wt %, or in the range from 0.02 wt %-1.0 wt %, or inthe range from 0.05 wt %-0.80 wt %, or in the range from 0.10 wt %-0.70wt %, or in the range from 0.15 wt %-0.60 wt %, or in the range from0.20 wt %-0.50 wt %.

In one embodiment, Cl⁻ is the only halide present in the glass. Theconcentration of Cl⁻ as the sole halide present in the glass may be inthe range from 0.01 wt %-0.80 wt %, or in the range from 0.01 wt %-0.60wt %, or in the range from 0.01 wt %-0.50 wt %, or in the range from0.05 wt %-0.50 wt %.

In another embodiment, Cl⁻ and Br⁻ are the only halides present in theglass. The concentration of Cl⁻ in the glass may be in the range from0.01 wt %-0.60 wt % and the concentration of Br⁻ may be in the rangefrom 0.01 wt %-0.70 wt %, or the concentration of Cl⁻ in the glass maybe in the range from 0.01 wt %-0.50 wt % and the concentration of Br⁻may be in the range from 0.01 wt %-0.60 wt %, or the concentration ofCl⁻ in the glass may be in the range from 0.01 wt %-0.40 wt % and theconcentration of Br⁻ may be in the range from 0.01 wt %-0.50 wt %.

The nanocrystalline cuprous halide phase may optionally include silver(Ag). The concentration of Ag in the glass may be in the range from 0.01wt %-0.20 wt %, or in the range from 0.01 wt %-0.15 wt %, or in therange from 0.01 wt %-0.10 wt %, or in the range from 0.005 wt %-0.05 wt%, or in the range from 0.005 wt %-0.025 wt %, or in the range from0.005 wt %-0.015 wt %. At least a portion of the total silver content ofthe photochromic material is present in the nanocrystalline cuproushalide phase. Some silver may be present in the base glass portion ofthe photochromic material. In one embodiment, essentially all of thesilver is present in the nanocrystalline cuprous halide phase. Silvermay be incorporated in the photochromic material by including silveroxide (AgO or Ag₂O) or silver nitrate (AgNO₃) in the batch compositionand is predominantly present as Ag⁺ in the glass.

The photochromic agent optionally includes tungsten and/or molybdenum.Tungsten may be incorporated by including a tungsten oxide (e.g. WO₃) inthe batch composition. Molybdenum may be incorporated by including amolybdenum oxide (e.g. MoO₃) in the batch composition. The concentrationof WO₃ in the photochromic glass may be in the range from 0 wt %-1.0 wt%, or in the range from 0.01 wt %-0.5 wt %, or in the range from 0.01 wt%-0.3 wt %, or in the range from 0.05 wt %-0.5 wt %, or in the rangefrom 0.10 wt %-0.25 wt %. The concentration of MoO₃ in the photochromicglass may be in the range from 0 wt %-1.0 wt %, or in the range from0.01 wt %-0.5 wt %, or in the range from 0.01 wt %-0.3 wt %, or in therange from 0.05 wt %-0.5 wt %, or in the range from 0.10 wt %-0.25 wt %.The combined concentration of WO₃ and MoO₃ in the photochromic glass maybe in the range from 0 wt %-1.0 wt %, or in the range from 0.05 wt %-0.5wt %, or in the range from 0.10 wt %-0.25 wt %.

The photochromic agent provides a sharp cutoff. The sharp cutoff mayoccur at a UV wavelength or at a wavelength in the short wavelengthportion of the visible portion of the spectrum. Under processingconditions described hereinbelow, at least a portion of the copper andhalide components precipitate in the form of nanocrystals in the glass.The nanocrystals include copper in the form of cuprous ion (Cu⁺) andhalide ion(s). The sharp cutoff is due to an excitonic absorption bandassociated with crystalline cuprous halides. The excitonic absorptionband is at or near the bandedge of the cuprous halide nanocrystals andfeatures a sharp absorption edge near 400 nm. Wavelengths shorter thanthe wavelength of the absorption edge are strongly absorbed, whilewavelengths longer than the wavelength of the absorption edge aretransmitted with high efficiency.

The sharp cutoff is characterized as a steep rise in absorption in thedirection of decreasing wavelength from the visible to the UV. The steeprise may be quantified as the slope of the absorption edge, whichcorresponds to the rate of change of the absorption coefficient(expressed in units of cm⁻¹) with respect to wavelength (expressed inunits of nm). The slope of the absorption edge of the present glassesmay be at least 0.10 cm⁻¹/nm, or at least 0.20 cm⁻¹/nm, at least 0.30cm⁻¹/nm, at least 0.40 cm⁻¹/nm, or in the range from 0.10 cm⁻¹/nm-1.0cm⁻¹/nm, or in the range from 0.15 cm⁻¹/nm-0.80 cm⁻¹/nm, or in the rangefrom 0.20 cm⁻¹/nm-0.70 cm⁻¹/nm, or in the range from 0.30 cm⁻¹/nm-0.60cm⁻¹/nm. The slope of the absorption edge may be expressed herein as theslope of the absorption edge at a selected wavelength. The selectedwavelength may be a wavelength less than 420 nm, or less than 410 nm, orless than 400 nm, or less than 390 nm, or in the range from 370 nm-420nm, or in the range from 380 nm-410 nm, or in the range from 390 nm-405nm.

The cutoff wavelength is defined herein as the wavelength at whichtransmission through a 2 mm thick sample of the glass is 1%. The cutoffwavelength is a wavelength in the UV or the short wavelength portion ofthe visible. The cutoff wavelength may be a wavelength less than orequal to 420 nm, or less than or equal to 415 nm, or less than or equalto 410 nm, or less than or equal to 405 nm, or less than or equal to 400nm, or less than or equal to 395 nm, or less than or equal to 390 nm, orin the range from 370 nm-420 nm, or in the range from 380 nm-410 nm, orin the range from 390 nm-405 nm.

Wavelengths longer than the cutoff wavelength are transmitted with highefficiency through the glass and wavelengths shorter than the cutoffwavelength are transmitted with low efficiency through the glass.Wavelengths longer than the cutoff wavelength may be referred to hereinas wavelengths above the cutoff wavelength. Wavelengths shorter than thecutoff wavelength may be referred to herein as wavelengths below thecutoff wavelength.

Transmission through a 2 mm thick sample of the glass may be 99% orgreater for a wavelength range extending from the cutoff wavelength to25 nm above the cutoff wavelength. Transmission through a 2 mm thicksample of the glass may be 99% or greater for a wavelength rangeextending from the cutoff wavelength to 50 nm above the cutoffwavelength. Transmission through a 2 mm thick sample of the glass may be99% or greater for a wavelength range extending from the cutoffwavelength to 100 nm above the cutoff wavelength. Transmission through a2 mm thick sample of the glass may be 99% or greater for a wavelengthrange extending from the cutoff wavelength to 200 nm above the cutoffwavelength.

Transmission through a 2 mm thick sample of the glass may be 1% or lessfor a wavelength range extending from the cutoff wavelength to 25 nmbelow the cutoff wavelength. Transmission through a 2 mm thick sample ofthe glass may be 1% or less for a wavelength range extending from thecutoff wavelength to 50 nm below the cutoff wavelength. Transmissionthrough a 2 mm thick sample of the glass may be 1% or less for awavelength range extending from the cutoff wavelength to 75 nm below thecutoff wavelength. Transmission through a 2 mm thick sample of the glassmay be 1% or less for a wavelength range extending from the cutoffwavelength to 100 nm below the cutoff wavelength.

In one embodiment, the photochromic glass is uncolored in the absence ofa photochromic response. As used herein, uncolored means that a viewerwith average vision perceives no color when looking through a sample ofthe glass (in its non-photochromic state) having a thickness of 2 mm. Inthis embodiment, the cutoff wavelength is positioned so that visiblewavelengths transmit with high efficiency (≥99% transmittance) throughthe glass (in its non-photochromic state), while UV wavelengths transmitwith low efficiency (≤1% transmittance) through the glass (in itsnon-photochromic state).

As described more fully hereinbelow, the spectral position of theabsorption edge may be varied by controlling the composition of thecuprous halide nanocrystals. Selection of the halide component, forexample, influences the spectral position of the absorption edge. Theabsorption edge of cuprous chloride nanocrystals occurs at shorterwavelength than the absorption edge of cuprous bromide nanocrystals.Intermediate positions of the absorption edge can be obtained byincluding a combination of chloride and bromide in the cuprous halidenanocrystals, where the absorption edge shifts to longer wavelengths asthe bromide content of the nanocrystals increases relative to thechloride content of the nanocrystals.

While not wishing to be bound by theory, it is believed that the cuproushalide nanocrystals of the present glasses include Cu²⁺. It is furtherbelieved that the presence of Cu²⁺ in the cuprous halide nanocrystalsprovides an absorption feature that enables initiation of a photochromicresponse from the cuprous halide nanocrystals upon exposure to visiblelight. The absorption feature occurs in the visible portion of thespectrum at an energy less than the energy of the absorption edgeassociated with the excitonic absorption. The absorption feature isabsent in the non-photochromic cuprous halide glasses described in U.S.Pat. No. 5,281,562. The present glasses demonstrate that proper controlof the oxidation state of copper in the glass and the overall oxidationstate of the glass enables incorporation of Cu²⁺ in the cuprous halidenanocrystals and that the presence of Cu²⁺ in the cuprous halidenanocrystals enables the cuprous halide nanocrystals to exhibit aphotochromic response while preserving the sharp cutoff in the UV orshort wavelength visible portions of the spectrum associated withcuprous halide nanocrystals that lack Cu²⁺.

FIG. 1 shows absorption spectra for a glass having cuprous halidenanocrystals and a glass of similar composition lacking cuprous halidenanocrystals. Spectrum 10 shows the spectrum of a 2 mm thick sample ofglass having the composition 77.05 wt % SiO₂, 2.16 wt % Al₂O₃, 14.6 wt %B₂O₃, 4.5 wt % Na₂O, 0.44 wt % SnO₂, 0.28 wt % CuO, 0.14 wt % Cl⁻, and0.057 wt % Br⁻. Spectrum 10 indicates high transmission in the visibleand the presence of a steep absorption edge near 400 nm. The steepabsorption edge indicates the presence of cuprous halide nanocrystals inthe glass and corresponds to an excitonic absorption process in thecuprous halide nanocrystals. Spectrum 20 shows the spectrum of a 2 mmthick glass sample lacking cuprous halide nanocrystals. The glass hadthe composition 54.35 wt % SiO₂, 6.87 wt % Al₂O₃, 28.6 wt % B₂O₃, 7.77wt % Na₂O, 0.10 wt % SnO₂, 1.20 wt % CuO, 0.18 wt % WO₃, and 0.023 wt %Ag. The composition included Cu, but lacked halides. Spectrum 20indicates high transmission in the visible and gradually increasingabsorption in the UV. The broad UV absorption band observed in spectrum20 corresponds to bandgap absorption of the glass. The absence ofcuprous halide nanocrystals in the glass precludes observation of thesharp excitonic absorption feature observed in spectrum 10. The glassexhibiting spectrum 10 provides superior cutoff performance relative tothe glass exhibiting spectrum 20. Although the glass exhibiting spectrum20 absorbs UV radiation, it does so incompletely and appreciable amountsof UV intensity pass through the glass.

Spectrum 30 shows the spectrum of a 2 mm thick glass sample of arepresentative base glass. The glass had the composition 56.14 wt %SiO₂, 6.01 wt % Al₂O₃, 29.9 wt % B₂O₃, 8.38 wt % Na₂O, and 0.12 wt %SnO₂. The glass lacks Cu and lacks halides. Spectrum 30 indicates thatthe glass does not exhibit a sharp UV cutoff and transmits a significantportion of the UV spectrum. The bandedge absorption is shifted tosignificantly shorter wavelengths relative to the glasses described byspectrum 10 and spectrum 20. The results indicate that inclusion of Cushifts the absorption edge to longer wavelength and that furtherinclusion of halide, within certain compositional and preparationconditions, provides a sharp cutoff wavelength.

Although the glass exhibiting spectrum 10 exhibits a sharp cutoff, it isnot photochromic. The glasses herein feature both a sharp cutoff and aphotochromic response. FIG. 2 shows the spectrum of representativeglasses in accordance with the present disclosure. Spectrum 40 shows thespectrum of a 2 mm thick sample of a glass having the composition 54.58wt % SiO₂, 6.91 wt % Al₂O₃, 27.70 wt % B₂O₃, 7.8 wt % Na₂O, 0.11 wt %SnO₂, 1.12 wt % CuO, 0.02 wt % Ag, 0.132 wt % Cl⁻, and 0.321 wt % Br⁻.The glass includes cuprous halide nanocrystals and its spectrum exhibitsa sharp excitonic absorption edge near 400 nm. The glass associated withspectrum 40 also exhibits an additional absorption feature atwavelengths extending beyond the excitonic absorption band into thevisible (up to ˜500 nm). The additional absorption feature is believedto be attributable to the presence of Cu²⁺ in the cuprous halidenanocrystalline phase of the glass. The presence of Cu²⁺ is evidenced bya slight green tint of the glass and the presence of an absorption bandnear 800 nm (not shown) that is consistent with known crystal fieldtransitions of Cu²⁺.

Spectrum 50 shows the spectrum of a 2 mm thick sample of a glass havingthe composition 54.43 wt % SiO₂, 6.90 wt % Al₂O₃, 28.0 wt % B₂O₃, 7.74wt % Na₂O, 0.10 wt % SnO₂, 1.12 wt % CuO, 0.17 wt % WO₃, 0.019 wt % Ag,0.132 wt % Cl⁻, and 0.325 wt % Br⁻. Except for inclusion of WO₃, theglass exhibiting spectrum 50 is compositionally similar to the glassexhibiting spectrum 40. Inclusion of WO₃ in the glass leads to anenhancement of absorption in the visible portion of the spectrum inaddition to the absorption due to the presence Cu²⁺ in the cuproushalide nanocrystals. The sharp UV cutoff near 400 nm is retained.

Without wishing to be bound by theory, the presence of Cu²⁺ in thecuprous halide nanocrystalline phase is believed to introduce defectstates into the bandgap of the cuprous halide nanocrystals. The presenceof defect states in the bandgap provides additional states between whichabsorption transitions can occur. Such absorption transitions wouldoccur at energies below the bandgap energy and hence at wavelengthslonger than the absorption transitions associated with bandedgeabsorption or excitonic absorption. It is believed that the Cu²⁺-relateddefect absorption band observed in FIG. 2 originates from defect statespresent in the bandgap of the cuprous halide nanocrystals. When Cu²⁺ isincorporated into cuprous halide nanocrystals, it occupies a Cu⁺ site.In order to preserve charge neutrality, one Cu²⁺ ion enters thestructure and two Cu⁺ ions are removed. The Cu²⁺ ion occupies one of theCu⁺ sites and the other Cu⁺ site is vacant. The vacancy and Cu²⁺ ionrepresent perturbations in the structure of the cuprous halidenanocrystal. The perturbations interrupt periodicity and produce statesin the bandgap. The vacancy associated with the missing Cu⁺ ion is acation vacancy. Absence of the Cu⁺ ion is believed to lower the bindingenergy of electrons on Cl⁻ ions neighboring the vacancy to producedefect-related energy states positioned at energies slightly above thevalence band edge of the cuprous halide nanocrystals. Similarly, Cu²⁺ iselectron deficient relative to the Cu⁺ ion that it replaces and exerts astronger attractive force on neighboring Cl⁻ ions to producedefect-related energy states positioned at energies slightly below theconduction band edge of the cuprous halide nanocrystals.

The net effect of incorporation of Cu²⁺ in the cuprous halidenanocrystals is the introduction of one or more defect-related energystates in the bandgap of cuprous halide. The defect-related energystates may be positioned in the band gap near the conduction bandedgeand/or the valence bandedge. Optical transitions between defect states,between states in the valence band and a defect state, or between adefect state and the conduction band occur at energies below the energyof the band gap and at energies below the energy of the excitonicabsorption. One or more transitions of these types contribute to theCu²⁺-related defect absorption observed in the visible in the presentphotochromic glasses.

The cuprous halide nanocrystalline phase may absorb in the visible atwavelengths less than 550 nm, or less than 525 nm, or less than 500 nm,or less than 475 nm, or less than 450 nm, or in the range from 400nm-575 nm, or in the range from 400 nm-550 nm, or in the range from 400nm-525 nm.

The presence of the Cu²⁺-related defect absorption feature in thevisible is believed to enable a photochromic response in the presentglasses. The mechanism of the photochromic response includes a reductionof Cu⁺ to Cu⁰ and aggregation of Cu⁰ ions within or on the surface ofthe cuprous halide nanocrystals to form a Cu⁰ nanophase. The Cu⁰nanophase has a surface plasmon that broadly absorbs in the visible toprovide a photodarkening effect. The Cu²⁺-related defect absorption inthe visible provides a mechanism for introducing the energy needed toinduce a photochromic response in the cuprous halide nanocrystals byproviding the energy needed to effect photoreduction of Cu⁺ to Cu⁰. Inglasses having cuprous halide nanocrystals that lack a photochromicresponse, the Cu²⁺-related defect feature is not observed and there isno mechanism through which visible light can introduce the energynecessary to drive photoreduction of Cu⁺ to Cu⁰. The non-photochromiccuprous halide glasses may or may not include Cu²⁺. If present, however,any Cu²⁺ in the non-photochromic cuprous halide glasses exists in theglass phase and not the nanocrystalline cuprous halide phase.

The present glasses can be prepared by batch melting the constituentoxide and halide compounds in proportions consistent with thecompositions described herein. The constituent components are combinedto form a batch composition and melted. Melting may be conducted incrucibles or other suitable vessels and the molten glass may be formedinto glass by conventional forming methods such as drawing, spinning,pressing, molding, cooling etc. The glass may optionally be annealed.

To impart a sharp cutoff in the UV or short wavelength visible portionof the spectrum to the glass, it is necessary to establish a cuproushalide nanocrystalline phase in the glass. To further insurephotochromic response upon visible excitation, it is necessary in oneembodiment for the cuprous halide nanocrystalline phase to include aneffective amount of Cu²⁺. The present glass compositions and heattreatment conditions establish an oxidation state for the glass thatenables formation of a cuprous halide nanocrystalline phase with asuitable concentration of Cu²⁺.

Heat treatment conditions favorable to the formation of cuprous halidenanocrystals include heating to 480° C. to 550° C. in an ambient of airand holding for at least 1 min, or at least 10 min, or at least 20 min,or at least 30 min, or at least 1 hr, or at least 2 hr, or at least 4hr, or at least 8 hr, or at least 12 hr, or at least 16 hr, or for atime in the range from 1 min-24 hr, or for a time in the range from 1min-12 hr, or for a time in the range from 5 min-24 hr, or for a time inthe range from 30 min-20 hr, or for a time in the range from 1 hr-16 hr,or for a time in the range from 1 hr-10 hr.

The heat treatment occurs after melting the batch composition. The batchcomposition may be cooled and solidified. Heat treatment as disclosedherein may occur during cooling of the batch composition as the batchcomposition cools from the melting conditions to room temperature beforeroom temperature is reached. Alternatively, heat treatment may occurafter the batch composition has been solidified or cooled to roomtemperature by subsequently reheating the solidified batch compositionto the heat treatment conditions disclosed herein.

FIGS. 3A-3C illustrate the photodarkening response of a representativeglass at different heat treatment conditions. The glass composition was:54.44 wt % SiO₂, 6.98 wt % Al₂O₃, 28.68 wt % B₂O₃, 7.70 wt % Na₂O, 0.93wt % CuO, 0.83 wt % As₂O₃, 0.168 wt % Cl⁻, and 0.19 wt % Br⁻. Thethickness of the glass was 2 mm. The glass was heat treated at 500° C.in air for 2 hr (FIG. 3A), 12 hr (FIG. 3B), and 24 hr (FIG. 3C). Thegraphs show transmission as a function of time of exposure to a broadband light source (1 kW HgXe lamp). For exposure times up to the timedemarcated by the dashed line in FIGS. 3A-3C, the light source wasfiltered with a 400 nm longpass filter and the glass was exposed only tolight having a wavelength longer than 400 nm. For exposure times longerthan the time demarcated by the dashed line in FIGS. 3A-3C, the longpassfilter was removed and the sample was exposed to the full spectrum ofthe light source.

The photodarkening response was characterized by measuring thetransmittance of a green laser beam through the sample. The intensity ofthe green laser beam was kept low to prevent the green laser fromaltering the photodarkening effect produced by the broadband lightsource. Before heat treatment, the glass exhibited high transmission at400 nm. Upon heat treatment a strong absorption edge is formed due tothe formation of cuprous halide nanocrystals. The absorption edge isevident upon heating for short times (e.g. FIG. 3A). The wavelength ofthe absorption edge is controlled by the Cl⁻ to Br⁻ ratio in the cuproushalide nanocrystals. Upon further heat treatment (e.g. FIGS. 3B and 3C),the presence of the cuprous halide phase induces photochromism, asevidenced by the decrease in transmittance observed when the glass wasilluminated with wavelengths of light greater than 400 nm.

FIG. 4 shows the absorption spectrum of two glasses that include copperand halide. Spectrum 60 was measured for a 2 mm thick glass having thecomposition 53.89 wt % SiO₂, 8.65 wt % Al₂O₃, 26.60 wt % B₂O₃, 8.64 wt %Na₂O, 0.96 wt % CuO, 0.71 wt % As₂O₃, 0.218 wt % Cl⁻, and 0.121 wt %Br⁻. Spectrum 70 was measured for a 2 m thick glass having thecomposition 51.22 wt % SiO₂, 21.02 wt % Al₂O₃, 10.40 wt % B₂O₃, 14.02 wt% Na₂O, 0.85 wt % CuO, 0.66 wt % As₂O₃, 0.286 wt % Cl⁻, and 0.121 wt %Br⁻. The spectra of FIG. 4 show that while both glasses contain copperand halides, only the glass having spectrum 60 exhibited an excitonabsorption feature that provides a sharp cutoff near 400 nm. The baseglass for the glass exhibiting spectrum 70 is outside the range thatpermits formation of copper halide nanocrystals upon heat treatment. Thelack of an excitonic absorption feature in spectrum 70 is due to theabsence of copper halide nanocrystals in the glass. The results of FIG.4 indicate that stabilization of copper halide nanocrystals from glassescontaining copper and halide components requires a base glass having asuitable composition. The mere presence of copper and halide(s) in aglass is insufficient to enable stabilization of a copper halidenanocrystalline phase.

FIG. 5 shows the absorption spectra of two glasses exhibiting excitonicabsorption. Spectrum 80 was measured for a 248 μm thick glass having thecomposition 54.21 wt % SiO₂, 6.89 wt % Al₂O₃, 28.73 wt % B₂O₃, 7.76 wt %Na₂O, 1.04 wt % CuO, 0.10 wt % SnO₂, 0.17 WO₃ wt %, 0.021 wt % Ag, 0.132wt % Cl⁻, and 0.316 wt % Br⁻. Spectrum 90 was measured for a 268 μmthick glass having the composition 54.26 wt % SiO₂, 6.90 wt % Al₂O₃,28.73 wt % B₂O₃, 7.86 wt % Na₂O, 0.94 wt % CuO, 0.10 wt % SnO₂, 0.17 WO₃wt %, 0.019 wt % Ag, 0.077 wt % Co₂O₃, 0.134 wt % Cl⁻, and 0.321 wt %Br⁻. Both glasses include copper, halides, Ag, and WO₃. The glass havingspectrum 90 also includes a low concentration of Co₂O₃. The resultsindicate that for glasses having a suitable base glass composition, theintroduction of Ag, WO₃ and colorants does not preclude formation ofcopper halide nanocrystals. The excitonic absorption feature is observedin both glasses.

FIGS. 6A and 6B show absorption spectra for a series of samples thatdiffer in halide composition of the cuprous halide nanocrystallinephase. The glasses have the compositions 92-97 listed in Table 1 belowand have spectra depicted respectively as traces 92-97 in FIGS. 6A and6B. Spectra for compositions 92, 93, and 94 are similar on the scaledepicted in FIG. 6A and are more easily distinguished in the enlargementshown in FIG. 6B.

TABLE 1 Composition (in wt %) of Glasses with Spectra depicted in FIGS.6A and 6B 92 93 94 95 96 97 SiO₂ 54.79 54.77 54.79 54.82 54.77 54.88Al₂O₃ 7.06 7.06 7.06 7.07 7.08 7.11 B₂O₃ 28.70 28.68 28.67 28.65 28.5928.52 Na₂O 8.07 8.05 8.08 8.04 8.01 8.03 CuO 0.88 0.89 0.86 0.86 0.830.76 As₂O₃ 0.65 0.64 0.64 0.64 0.63 0.64 Cl⁻ 0.337 0.301 0.271 0.2010.097 0.014 Br⁻ 0 0.051 0.103 0.218 0.374 0.525

The base glass composition is similar for compositions 92-97. Thedifference of interest is in the halide content. The Cl⁻ concentrationdecreases across the series of compositions 92-97 and the Br⁻concentration increases across the series of compositions 92-97. Each ofthe spectra exhibit an excitonic absorption feature, consistent with thepresence of a cuprous halide nanocrystalline phase. The results indicatethat as the Br⁻ concentration increases, the cutoff wavelengthassociated with the excitonic absorption feature shifts to longerwavelength. The shortest cutoff wavelength was observed for composition92 and progressively longer cutoff wavelengths were observed as the Br⁻concentration increased. FIG. 6B shows an enlargement of the spectra inthe vicinity of the cutoff wavelength. The trend is quantified in FIG.7, which shows the UV cutoff wavelength as a function of the ratio ofBr⁻ content (wt % Br normalized to the molecular weight of Br (79.9g/mol)) to total halide content (Br⁻+Cl⁻) (sum of wt % Br normalized tothe molecular weight of Br (79.9 g/mol) and wt % Cl normalized to themolecular weight of Cl (35.5 g/mol)) in the glass. The data points inFIG. 7 are labeled according to the compositions presented in Table 1and FIGS. 6A and 6B. The data demonstrate the ability to control thecutoff wavelength by varying the halide composition of the cuproushalide nanocrystalline phase.

In further embodiments, the present description extends to photochromicglasses having sharp UV cutoffs with a cuprous halide nanocrystallinephase that includes silver. The silver-containing cuprous halidenanocrystalline phase also includes Cu²⁺. Silver can be incorporated inthe glass composition by including a silver precursor, such as silveroxide (AgO or Ag₂O) or silver nitrate (AgNO₃), in the batch composition.Silver is primarily present as Ag⁺ in the glass and is reduced to Ag⁰ byphotoreduction during the photochromic transition.

It has been observed that incorporation of Ag in the cuprous halidenanocrystals enhances photochromic performance. FIGS. 8A and 8B show thephotodarkening response of two photochromic glasses. The photodarkeningresponse was measured as described hereinabove in connection with FIGS.3A-3C. The glass depicted in FIG. 8A had a thickness of 2 mm and thecomposition 54.21 wt % SiO₂, 6.89 wt % Al₂O₃, 28.73 wt % B₂O₃, 7.76 wt %Na₂O, 1.04 wt % CuO, 0.10 wt % SnO₂, 0.17 WO₃ wt %, 0.021 wt % Ag, 0.132wt % Cl⁻, and 0.316 wt % Br⁻ and included Ag in the cuprous halidenanocrystalline phase. The glass depicted in FIG. 8B had a thickness of2 mm and the composition 54.53 wt % SiO₂, 6.93 wt % Al₂O₃, 28.78 wt %B₂O₃, 7.91 wt % Na₂O, 1.07 wt % CuO, 0.11 wt % SnO₂, 0.149 wt % Cl⁻, and0.33 wt % Br⁻ and lacked Ag in the cuprous halide nanocrystalline phase.The results show a significant enhancement in the rate of photodarkeningfor the glass when Ag was included in the composition. The presence ofWO₃ in the composition also facilitated the photodarkening process. Thepeak near 2.5 min in FIG. 8B shows fading as the illuminating lamp wascovered for 30 s.

FIGS. 9A-9C illustrate the effect of temperature on the photodarkeningresponse of a photochromic glass that contains a cuprous halidenanocrystalline phase with Ag. The glass composition was 54.21 wt %SiO₂, 6.88 wt % Al₂O₃, 28.73 wt % B₂O₃, 7.73 wt % Na₂O, 1.14 wt % CuO,0.10 wt % SnO₂, 0.18 WO₃ wt %, 0.022 wt % Ag, 0.131 wt % Cl⁻, and 0.34wt % The thickness of the glass was 2 mm. The photodarkening responsewas measured as described hereinabove in connection with FIGS. 3A-3C.FIGS. 9A, 9B, and 9C show the response at 72° F., 122° F., and 160° F.,respectively. As temperature increases, the photodarkening responseweakens. An Arrhenius plot of the temperature dependence of theabsorption (expressed in units of mm⁻¹) associated with thephotodarkening response as a function of 1000/T (expressed in units ofK⁻¹) is presented in FIG. 9D. The absorption reported is the absorptionafter an exposure of the glass to the white light for 5 min. TheArrhenius plot shows a linear dependence on reciprocal temperature,which is consistent with a diffusion-controlled process for themechanism underlying the photodarkening response.

FIGS. 10A and 10B show the differential absorbance spectra associatedwith photodarkening for glasses having a nanocrystalline cuprous halidephase with and without Ag, respectively. The glass exhibiting thespectra shown in FIG. 10A had the composition 54.22 wt % SiO₂, 6.93 wt %Al₂O₃, 28.69 wt % B₂O₃, 7.76 wt % Na₂O, 0.94 wt % CuO, 0.87 wt % As₂O₃,0.021 wt % Ag, 0.235 wt % Cl⁻, and 0.12 wt % Br⁻ and the glassexhibiting the spectra shown in FIG. 10B had the composition 54.01 wt %SiO₂, 6.90 wt % Al₂O₃, 28.66 wt % B₂O₃, 7.76 wt % Na₂O, 0.97 wt % CuO,0.80 wt % As₂O₃, 0.18 WO₃ wt %, 0.242 wt % Cl⁻, and 0.134 wt % Br⁻. Thethickness of the glass samples was 2 mm.

The absorbance associated with the differential absorbance bands shownin FIGS. 10A and 10B are responsible for the darkening observed in theglass upon exposure to visible light. The glasses were placed in aspectrophotometer and photodarkened by exposing them to a blue laser(405 nm). The glass samples were exposed to the blue laser until a darkspot was evident on the glass. The blue laser was then removed and aseries of difference spectra as a function of time, relative to the timeof removal of the blue laser, was obtained. The difference spectra showthe fading response of the absorption transition associated withphotodarkening. The abscissa of the difference spectra shown in FIGS.10A and 10B, Δ(absorption), corresponds to the change in absorbance ofthe glass in a photodarkened state relative to the glass in its original(non-photodarkened) state. The difference spectra having maximumintensity in FIGS. 10A and 10B correspond to the difference inabsorption of the glasses relative to its non-darkened state in thefirst measurement scan following removal of the blue laser. Theremaining spectra show the fading of the photo-induced absorption as theglass returns to its original non-darkened state. The series of spectrawere obtained as consecutive spectral scans following removal of thelight source. The times (expressed as hours:minutes:seconds) listed inthe legends of—FIGS. 10A and 10B correspond to the start times of theconsecutive spectral scans. It is understood that the scans require afinite amount of time and some fading may occur during the scan so thatthe difference spectra may correspond to an average over the measurementtime of the scan. The spectra are ordered (from high intensity to lowintensity) in increasing time following removal of the blue laser. Astime increases, a greater degree of fading of absorption intensity isobserved as the photodarkening response reverses and the glasses returnto their original non-darkened state.

A noteworthy feature of the spectra presented in FIGS. 10A and 10B isthe difference in the wavelengths of the absorption features for theglass with Ag in the nanocrystalline cuprous halide phase and the glasswithout Ag in the nanocrystalline cuprous halide phase. The glasswithout Ag in the nanocrystalline cuprous halide phase (FIG. 10B)exhibits a strong absorption peak near 600 nm that is not apparent inthe spectrum of the glass with Ag in the nanocrystalline cuprous halidephase (FIG. 10A). The spectral feature observed near 600 nm isconsistent with photoreduction of Cu⁺ to Cu⁰ during the photochromictransition to the darkened state and corresponds to a plasmon absorptionassociated with an aggregated Cu⁰ nanophase at or near the surface ofthe cuprous halide nanocrystals. The primary absorption feature of theglass with Ag in the nanocrystalline cuprous halide phase occurs near500 nm. This feature is consistent with a known plasmon absorption bandassociated with an aggregated Ag⁰ nanophase in classical silver halidephotochromic glasses. The results indicate that a preferential reductionof Ag occurs upon photoexcitation of Ag-containing cuprous halidenanocrystalline phases. The extinction coefficient of the Ag⁰ plasmonabsorption transition is higher than the extinction coefficient of theCu⁰ plasmon absorption transition, which is consistent with the enhancedphotodarkening effect noted in FIGS. 8A and 8B.

A photochromic glass with a nanocrystalline cuprous halide phase thatcontains Ag may have a peak absorption at a wavelength in the range from390 nm-540 nm, or in the range from 450 nm-530 nm, or in the range from460 nm-520 nm. A photochromic glass with a nanocrystalline cuproushalide phase that contains Ag and Cu²⁺ may have a peak absorption at awavelength in the range from 390 nm-540 nm, or in the range from 450nm-530 nm, or in the range from 460 nm-520 nm.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the illustrated embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments that incorporate the spirit and substance of the illustratedembodiments may occur to persons skilled in the art, the descriptionshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of making a photochromic glasscomprising: melting a batch composition, said batch compositionincluding 19 wt %-39 wt % B₂O₃, 0.5 wt %-15 wt % Al₂O₃, 46 wt %-65 wt %SiO₂, 3.5 wt %-12.5 wt % alkali metal oxide, 0.40 wt %-2.5 wt % CuO, and0.01 wt %-1.3 wt % halide.
 2. The method of claim 1, wherein said batchcomposition including 22 wt %-35 wt % B₂O₃, 1.0 wt %-11 wt % Al₂O₃, 50wt %-61 wt % SiO₂, 4.5 wt %-10.5 wt % alkali metal oxide, 0.60 wt %-1.8wt % CuO, and 0.10 wt %-0.60 wt % halide.
 3. The method of claim 1,wherein said halide includes Cl.
 4. The method of claim 3, said batchcomposition includes 0.01 wt %-0.80 wt % of said halide.
 5. The methodof claim 3, said batch composition includes 0.05 wt %-0.50 wt % of saidhalide.
 6. The method of claim 1, wherein said halide includes Cl andBr.
 7. The method of claim 6, wherein said batch composition includes0.01 wt %-0.60 wt % of said Cl and 0.01 wt %-0.70 wt % of said Br. 8.The method of claim 6, wherein said batch composition includes 0.01 wt%-0.40 wt % of said Cl and 0.01 wt %-0.50 wt % of said Br.
 9. The methodof claim 1, wherein said batch composition further includes a redoxagent.
 10. The method of claim 9, wherein said redox agent comprises Sn.11. The method of claim 10, wherein said batch composition includes upto 0.80 wt % of said redox agent.
 12. The method of claim 1, whereinsaid batch composition further comprises silver oxide.
 13. The method ofclaim 1, further comprising cooling said batch composition, said coolingoccurring after said melting.
 14. The method of claim 13, wherein saidcooling includes maintaining said batch composition at a temperature inthe range from 480° C.-550° C. for at least 1 hour.
 15. The method ofclaim 13, wherein said cooling includes maintaining said batchcomposition at a temperature in the range from 480° C.-550° C. for atleast 4 hours.
 16. The method of claim 13, wherein said cooling includesmaintaining said batch composition at a temperature in the range from480° C.-550° C. for at least 8 hours.
 17. The method of claim 13,wherein said cooling solidifies said batch composition to form a glass.18. The method of claim 17, further comprising heating said glass, saidheating including maintaining said glass at a temperature in the rangefrom 480° C.-550° C. for a time in the range from 1 min to 24 hour. 19.The method of claim 18, wherein heating includes maintaining said glassat a temperature in the range from 480° C.-550° C. for a time in therange from 1 min-12 hours.
 20. The method of claim 18, wherein heatingincludes maintaining said glass at a temperature in the range from 480°C.-550° C. for a time in the range from 1 hour-10 hours.